1. Field of the Invention
The present invention relates to an ultrasonic temperature measuring apparatus employing ultrasonic waves to measure the average temperature of a medium, and particularly, to an ultrasonic temperature measuring apparatus that correctly calculates the propagation time of ultrasonic waves propagated for a predetermined distance through a medium, and according to the propagation time, correctly determines the average temperature of the medium.
2. Description of the Related Art
The velocity of sound waves propagated through a medium depends on the temperature of the medium. According to this fact, the temperature of the medium is measurable. The velocity V [m/sec] of sound waves propagated through the medium and the temperature T [.degree.K.] of the medium have the following relationship: EQU V=331.45+0.605T [m/sec] (1)
When the sound waves are propagated for a predetermined distance L through the medium, the sound velocity V is calculable as follows, if a propagation time t of the sound waves for the distance L is measurable: EQU V=L/t [m/sec] (2)
The temperature T of the medium is, therefore, expressed as follows: EQU T=(L/t-331.45)/0.605 (3)
An example of an ultrasonic temperature measuring apparatus is disclosed in Japanese Unexamined Utility Model Publication No. 63-39646. The apparatus employs a transmitter for transmitting ultrasonic waves through a medium toward a receiver spaced apart from the transmitter by a given distance. The apparatus measures a propagation time of the ultrasonic waves between the transmitter and the receiver, and according to the propagation time, determines the average temperature of the medium.
Another example of such a measuring apparatus is disclosed in Japanese Unexamined Patent Publication No. 54-102181. This disclosure employs a transmitter and two receivers spaced apart from each other by a given distance along an ultrasonic propagation path in a medium. The transmitter emits ultrasonic waves, which pass through the medium and are received by the receivers. The apparatus measures a phase difference between the emitted ultrasonic waves and the received ultrasonic waves and converts the phase difference into time. According to the time, the apparatus calculates the average temperature of the medium.
The former apparatus is capable of operating over a long distance for ultrasonic propagation. Accordingly, this apparatus can measure a wide range of temperatures over a wide area of the medium. The propagation time measured by this apparatus, however, fluctuates depending on a level of received ultrasonic waves, so that the temperature measured by this apparatus is inaccurate.
The latter apparatus can accurately measure the temperature, because the phases of the ultrasonic waves are not affected by a level of the received ultrasonic waves. A detectable phase difference range of this apparatus, however, is narrow. For example, the phase difference is within only one period of ultrasonic waves. Accordingly, this apparatus is unable to measure a wide range of temperatures over a wide area of a medium.